As a fixing device to be mounted on a copying machine or a printer, there is available a device having an endless belt, a ceramic heater which contacts an inner surface of the endless belt, and a pressure roller which forms a fixing nip portion together with the ceramic heater via the endless belt. When small size sheets are continuously printed by an image forming apparatus that mounts thereon the fixing device, there occurs a phenomenon (non-sheet-passing part temperature rise) in which a temperature of a region through which the sheets are not passed in a lengthwise direction of fixing nip portion gradually rises. When the temperature in the non-sheet-passing part becomes too high, it may cause damages to respective parts of the device, or when printing is performed on large size sheets while the non-sheet-passing part temperature rise is occurring, toner may be subjected to high-temperature offset in a region corresponding to the non-sheet-passing part of the small size sheets.
As one of approaches for inhibiting the non-sheet-passing part temperature rise, there is a possible idea of forming the heat-generation resistors on a ceramic substrate with material having positive resistance-temperature characteristics, and arranging two electro-conductive elements on both ends in the widthwise direction of substrate so that electric current flows through the heat-generation resistors in the widthwise direction of heater (in a conveyance direction of recording sheets). The concept is such that when the non-sheet-passing part undergoes a temperature rise, resistance values of the heat-generation resistors of the non-sheet-passing part are decreased, and electric current which flow through the heat-generation resistors of the non-sheet-passing part is inhibited, thereby inhibiting heat generation of the non-sheet-passing part. Positive resistance-temperature characteristics imply characteristics in which an electrical resistance increases when temperature is raised. This is hereinafter referred to as positive temperature coefficient (PTC).
Japanese Patent Application Laid-Open No. 2005-209493 discusses a method for arranging two electro-conductive elements at both ends in a widthwise direction of substrate so that electric current flows in a widthwise direction of a heater (in a conveyance direction of recording sheets), using material having positive resistance-temperature characteristics. It was found that, with this method, when a temperature distribution occurs in the widthwise direction of heater (in a conveying direction of sheets), resistance values of the heat-generation resistors arranged in a high-temperature part in the widthwise direction increase, and as a result, heat-generation amounts of the high-temperature part in the widthwise direction increase, eventually the temperature distribution in the widthwise direction is likely to become non-uniform.